The present invention relates to a charge quenching apparatus for quenching unnecessary charges on a photoconductor of an electrophotographic copying machine when making copies and more particularly to a charge quenching apparatus for an electrophotographic copying machine capable of making copies with unit magnification and variable magnification.
In a convention electrophotographic copying machine of the above-mentioned type, the surface of a photoconductor is uniformly charged and exposed to the optical image of an original to form a latent electrostatic image on the photoconductor, corresponding to the optical image, and the latent electrostatic image is developed by a developer, rendering the image visible.
In a conventional electrophotographic copying machine, when an original is copied with unit magnification, using a transfer sheet larger than the original, the area of the transfer sheet in excess of the size of the original can be rendered pure white if the original placed on the contact glass of the electrophotographic copying machine is covered closely by an original pressure plate with a white contact surface. In that case, light from an exposure lamp reflected from the white contact surface reaches the outside area of the photoconductor, around the area corresponding to the original, thus completely quenching the unnecessary charges in that outside area, so that a copy of a larger sheet than the original, with a blank margin, can be obtained.
However, when the contact surface of the original pressure plate is darker than the background of the original, or happens to be dirty or stained, an inevitable occurs in practice, or when the original placed on the contact glass is a book or the like and the original pressure plate cannot be closed completely, the unnecessary charges in the outside area of the photoconductor around the area corresponding to the original will not be quenched completely, so that toner will be deposited there to some degree, and copies with dirty or dark margins will be obtained.
Obviously, such margins should be eliminated from the copies as much as possible.
In addition, in unit magnification, in a conventional electrophotographic copying machine of the type in which the maximum size of the latent electrostatic image to be formed on the photoconductor is not determined in accordance with the size of the transfer sheet selected, i.e., when the maximum possible size of the photoconductor is always initially charged, then when a transfer sheet smaller than the maximum is used, a latent electrostatic image as large as the maximum-latent-image size is nevertheless formed on the photoconductor and developed, with only a part of the developed toner image of a size equal to that of the transfer sheet being transferred to the transfer sheet. Therefore, the unnecessary toner deposited outside the area corresponding to the transfer sheet has to be removed from the photoconductor. As a result, toner is wasted and excess load is applied to the cleaning apparatus when cleaning the photoconductor.
Further, when a copy reduced in size in comparison with the original is made with variable magnification, the maximum latent electrostatic image area on the photoconductor, which may corrspond to the whole surface of the contact glass covered by the original pressure plate, is also reduced in size, and charges in the non-image area outside the maximum latent electrostatic image area remain on the photoconductor without being quenched, as if such outside area were pure black since no light reaches the outside area. Therefore, when development is performed, a large quantity of toner is deposited in that area, is wasted, and must be removed, with excess load applied to the cleaning apparatus of the copying machine.